The movie, directed by Olivier Hirschbiegel, covers the final years of Diana's life, mainly focusing on her romance with Khan after her divorce from Prince Charles.

In an interview with Manhattan Magazine, Watts said playing Diana was the greatest challenge she's ever tackled as an actor.

“It was the hardest thing I’ve done,” Watts says, “because of the pressure of everyone’s beliefs about who she was, and the fact that it was just really hard to claim her as my own, since everybody else feels they know her.”

There is currently no official U.S. release date for the film. But it is scheduled to open in European theaters in September.